This invention relates to the real time transmission of video information over a standard data bus in a personal computer.
A difficult problem has heretofore prevented the real-time delivery of digitized video data across a typical personal computer bus. The data transfer rate requirements of the digitized video, due to both raw bandwidth and color bit depth, approach the maximum data transfer bandwidth of the bus.
A number of schemes have been devised in the prior art to accomplish such video transmission over a computer bus. In one scheme, a large local memory is employed as a buffer to store several frames of video data, and the data is transferred gradually from these frames over the bus. Typically, one frame's worth of data is written to one memory storage area while another frame's worth of data is read from another memory storage area. Such a scheme allows complete frame transfers and distributes such transfers over the entire time span of a frame. However, this solution is often not feasible to implement for real-time capability, is intensive in the use of memory and typically has a high cost.
An alternate prior art video capture system requires the addition of a local video data bus of higher bandwidth, which transfers directly into a dedicated video frame buffer, not using the main computer bus. In this scheme, the main computer bus is not burdened with any overhead from the video transfer. However, the video can be displayed only on the portion of the computer's display system controlled by that frame buffer, and not on any arbitrary frame buffer in the computer system. Furthermore this system will work only with other hardware designed specifically for this video transfer scheme.
A third prior-art approach is to overlay the desired video image in the analog domain. This is similar to the "chroma keying" scheme used commonly in television newsrooms. In order to determine the size of the desired overlay, the video to be transferred is digitized, processed in the digital domain to fit within the desired window, converted back into analog and then superimposed over the video signal being sent to the display system of the computer. One disadvantage of this system is that it operates only with display hardware specially modified for this scheme. Another disadvantage is that such systems typically require interlaced (NTSC, "PAL" or "SECAM") rate display boards, which are ill-suited to the display of computer images without complex filter circuitry.